A 'proof' of God's existence - free will

[Brian37]

Which still leads me to WHY do we believe we have free will?

Yup.

What evolutionary benefit is there to thinking my actions are willful when they are not? 

There's evolutionary benefit to superstition, privately held or socially organized.

The two things are entirely unrelated.  The evolutionary benefit of an imaginary organ or ability is a non starter..but superstition and tradition are not imaginary organs or abilities.

Not in any real sense. The Ancient Egyptians were successful for 3,000 years grouping round belief in gods that were never real. 

My cat has a ritual of jumping on my side and digging his nose into my ear to wake me up to feed him in the morning. That benefits him in a survival sense and a social sense, but Tony the Tiger nor Tigger are real cats.

I wouldn't call it a "benefit" as much as I would call it a flaw in our evolution that has the side effect of causing social structures.